Cut the leeks and stir fry it with the butter and garlic. Add the diced sweet potatoes and the stock. Cook for about 20 minutes or untile potatoes are coocked. Blend or process the soup adding more or less stock accordingly to your tastes.Boil the remainin potatoes and mash them. Add butter, parsley, grates parmigiano chesse and beaten egg. Shape little dumplings and put aside.Return the soup to the saucepan and bring it to a boil. Shape little dumplings with two spoons and drop them into the soup. Let them cook for about 10 minutes or until they come back on the surface. Serve very warm with some more parmigiano and a drizzle of olive oil.